<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style>
    .parent{
        width: 20%;
        height: 500px;
        word-wrap:break-word ;
        display: flex;
        flex-flow: column wrap;
    }
    .firstChild{
        width: 100%;
        padding: 20px;
    }
    .lastChild{
        flex: 1 0 auto;
    }
</style>
<body>
    <div class="parent">
        <div class="firstChild">
            <p>asdfasdfjakfdssssssssssjddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd</p>
        </div>
        <div class="lastChild">asdffffffffffffffffffffffff</div>
    </div>
</body>
</html>